900 hp should be relatively easy with 600 cubic inches if the right person gets ahold of it or helps you choose the parts.
But flat top pistons in a BBC means no compression. With open chambered heads it takes a .340 high dome to even get above 10 to 1.
Most of the GOOD HEADS that are stock configuration are open chambered with around 118 cc chambers.I don't know what is available for heads that are good that have small chambers.With flat tops you will need really small chambers or a set of heads that are cut a bunch to get even decent compression.
I am speaking of a 454 your big bore/stroke motor would take less dome I'm sure to get the same compression.
jmo